The present invention relates to a display-integrated type tablet device for use in a personal computer, a word processor, and the like.
As a means for inputting a handwritten letter or a figure into a computer or a word processor, there has been put into practical use a display-integrated type tablet device which is formed by laminating an electrostatic induction type tablet on a liquid crystal display and is capable of receiving an input of a letter or a figure into its electrostatic induction type tablet as if the letter or figure were written on a paper by writing implements. However, in such a display-integrated type tablet device, electrodes are viewed as a grating on the display screen due to a difference in reflectance or transmission between a portion having an electrode and a portion having no electrode, which has been a cause of degrading the quality of an image displayed on the liquid crystal display screen.
As a tablet free of the above-mentioned drawback, lately the inventor of the present invention has proposed a display-integrated type tablet device as shown in FIG. 14 (Japanese Patent Laid-Open Publication No. HEI 5-53726 issued on Mar. 5, 1993).
The above-mentioned display-integrated type tablet device is not a prior art to be referred to, however, it is described for the purpose to allow the present invention to be more understandable.
The above-mentioned display-integrated type tablet device has electrodes which concurrently serve as image display electrodes of a liquid crystal display (LCD) and as coordinate detection electrodes of an electrostatic type tablet device. There are provided in one frame period a coordinate detection period when designated coordinates on the tablet are detected and an image display period when an image is displayed as shown in FIG. 15 to time-sharingly execute the coordinate detection and image display.
Referring to FIG. 14, an LCD panel 1 is constructed by interposing liquid crystals between common electrodes Y.sub.1 through Y.sub.n (an arbitrary common electrode represented by Y hereinafter) and segment electrodes X.sub.1 through X.sub.m (an arbitrary segment electrode represented by X hereinafter) which are arranged at right angles to each other, in which each portion where a common electrode Y and a segment electrode X intersect each other constitutes each pixel. In other words, n.times.m dot pixels are arranged in matrix in the LCD panel 1.
It is noted that the numbers n and m are determined to be n=480 and m=640 in a liquid crystal display panel for use in a personal computer with a monochrome display.
The above-mentioned display-integrated type tablet device has an advantage of permitting cost reduction as well as compact and light weight design by virtue of the concurrent use of the electrodes and drive circuits as those of the liquid crystal display and those of the electrostatic induction type tablet in addition to an advantage of making the grating-shaped electrodes invisible for a better image presentation in contrast to the conventional type formed by laminating the electrostatic induction type tablet on the liquid crystal display.
The above-mentioned display-integrated type tablet device operates in a manner as follows. A common electrode drive circuit 2 for driving the common electrode Y and a segment electrode drive circuit 3 for driving the segment electrode X are connected to a display control circuit 5 and a detection control circuit 6 via a switching circuit 4. The switching circuit 4 is controlled by a control circuit 7 so that it outputs an output signal from the display control circuit 5 to the common electrode drive circuit 2 and the segment electrode drive circuit 3 in an image display period and outputs an output from the detection control circuit 6 to the common electrode drive circuit 2 and the segment electrode drive circuit 3 in a coordinate detection period.
Although the switching circuit 4, the display control circuit 5, the detection control circuit 6, and the control circuit 7 are expressed as separated blocks in FIG. 14, the circuits are composed of an LSI (Large Scale Integrated) circuit in practice. Therefore, the LSI cannot be strictly sectioned into such blocks in a practical circuit arrangement.
In the image display period, firstly a mode signal (mode) output from the control circuit 7 to the segment electrode drive circuit 3 and the switching circuit 4 is switched to the image display mode. Then the segment electrode drive circuit 3 selects the image display mode, and the switching circuit 4 is switched so as to select the output signal from the display control circuit 5.
Then there are output, from the display control circuit 5, shift data s from a shift data output terminal S, an inverted signal fr from an inverted signal output terminal FR, a clock signal cp1 from a clock output terminal CP1, a clock signal cp2 from a clock output terminal CP2, and display data D.sub.0 through D.sub.3 from data output terminals D0 through D3.
The clock signal cp1 is a clock signal which has a period when pixels in one line display an image, and the signal is input as a clock signal cp1o to a clock input terminal YCK of the common electrode drive circuit 2 and a latch pulse input terminal XLP of the segment electrode drive circuit 3 via an output terminal CP1O of the switching circuit 4. The shift data s which is a pulse signal for selecting a specified common electrode Y is input as shift data so to a shift data input terminal DIO1 of the common electrode drive circuit 2 in synchronization with the clock signal cp1o via an output terminal SO of the switching circuit 4.
When the shift data so is input to the common electrode drive circuit 2, the pulse position of the shift data so is shifted in a shift register built in the common electrode drive circuit 2 in synchronization with the clock signal cp1o, and drive pulses of a common electrode drive signal are applied to the common electrodes Y.sub.1 through Y.sub.n from output terminals O1 through On of the common electrode drive circuit 2 in correspondence with the shift position. The common electrode drive signal is generated based on bias power sources V.sub.0 through V.sub.5 supplied from a DC power supply circuit 12.
The clock signal cp2 is a clock signal which has a period being a division of a period when pixels in one line displays an image, and the signal is input as a clock signal cp2o to a clock input terminal XCK of the segment electrode drive circuit 3 via an output terminal CP20 of the switching circuit 4.
The image display data D.sub.0 through D.sub.3 are input as display data D.sub.0 o through D.sub.3 o to input terminals XD0 through XD3 of the segment electrode drive circuit 3 via output terminals D0O through D3O of the switching circuit 4, and then successively taken into a register in the segment electrode drive circuit 3 in synchronization with the clock signal cp2o. When all the display image data corresponding to the pixels in one line are taken in, the display data taken in are latched at a timing of the clock signal cp1o input to the latch pulse input terminal XLP. Then drive pulses of the segment electrode drive signal corresponding to the display data are applied from output terminals O1 through Om of the segment electrode drive circuit 3 to the segment electrodes X.sub.1 through X.sub.m. The segment electrode drive signal is also generated based on the bias power sources V.sub.0 through V.sub.5 supplied from the DC power supply circuit 12.
It is noted that the inverted signal fr is a signal for preventing possible deterioration of the liquid crystals due to electrolysis by periodically inverting the direction of voltage application to the liquid crystals in the image display period. The inverted signal fr is input as an inverted signal fro to an inverted signal input terminal YFR of the common electrode drive circuit 2 and an inverted signal input terminal XFR of the segment electrode drive circuit 3 via an inverted signal output terminal FRO of the switching circuit 4.
Thus the pixel matrix of the LCD panel 1 is line-sequentially driven by the operations of the common electrode drive circuit 2 and the segment electrode drive circuit 3 to display an image corresponding to the display data D.sub.0 through D.sub.3 on the LCD panel 1.
In the coordinate detection period, firstly the mode signal (mode) output from the control circuit 7 to the segment electrode drive circuit 3 and the switching circuit 4 is switched to the coordinate detection mode. Then the segment electrode drive circuit 3 selects the coordinate detection mode, and the switching circuit 4 is switched so as to select the output signal from the detection control circuit 6.
Then there are output, from the detection control circuit 6, shift data sd from a shift data output terminal Sd, an inverted signal frd from an inverted signal output terminal FRd, a clock signal cp1d from a clock output terminal CP1d, a clock signal cp2d from a clock output terminal CP2d, and drive data D.sub.0 d through D.sub.3 d from data output terminals D0d through D3d.
The clock signal cp1d is a clock signal which has a period when one common electrode Y is scanned, and the signal is input as the clock signal cp1o to the clock input terminal YCK of the common-electrode drive circuit 2 and the latch pulse input terminal XLP of the segment electrode drive circuit 3 via the output terminal CP1O of the switching circuit 4. Meanwhile, the shift data sd which is a pulse signal for selecting a specified common electrode Y is input as the shift data (so) to the shift data input terminal DIO1 of the common electrode drive circuit 2 or a shift data input terminal EIO1 of the segment electrode drive circuit 3 via the output terminal SO of the switching circuit 4 in synchronization with the clock signal cp1d.
Then, in the same manner as in the image display period described above, the position of the shift data (so) input to the common electrode drive circuit 2 is shifted in the shift register of the common electrode drive circuit 2 in synchronization with the clock signal cp1o, and scanning pulses of common electrode drive signals Y.sub.1 through Y.sub.n (arbitrary common electrode scanning signal represented by y hereinafter) are successively applied from the output terminals O1 through On corresponding to the shift position to the common electrodes Y.sub.1 through Y.sub.n. The common electrode scanning signal y is generated based on the bias power sources V.sub.0 through V.sub.5 supplied from the DC power supply circuit 12.
The clock signal cp2d is a clock signal which has a period when the segment electrode X is scanned, and the signal is input as the clock signal cp2o to the clock input terminal XCK of the segment electrode drive circuit 3 via the output terminal CP20 of the switching circuit 4.
The drive data D.sub.0 d through D.sub.3 d are input as drive data D.sub.0 o through D.sub.3 o to the input terminals XD0 through XD3 of the segment electrode drive circuit 3 via the output terminals D0O through D3O of the switching circuit 4, and then successively taken into the register of the segment electrode drive circuit 3 in synchronization with the clock signal cp2o. Then scanning pulses of the segment electrode scanning signals x.sub.1 through x.sub.m (arbitrary segment electrode scanning signal represented by x hereinafter) corresponding to the drive data are output from the output terminals O1 through Om of the segment electrode drive circuit 3 to the segment electrodes X.sub.1 through X.sub.m. The segment electrode scanning signal x is also generated based on the bias power sources V.sub.0 through V.sub.5 supplied from the DC power supply circuit 12.
FIG. 16 is a timing chart of the scanning signals in the coordinate detection period of the display-integrated type tablet device. The coordinate detection period is separated into an x-coordinate detection period and a subsequent y-coordinate detection period. In the x-coordinate detection period, the segment electrode scanning signal x which is a pulse voltage signal is sequentially applied to the segment electrode X. In the y-coordinate detection period, the common electrode scanning signal y which is a pulse voltage signal is sequentially applied to the common electrode Y.
With the application of the pulse voltage signals, a voltage is induced at a designation coordinate detection pen (referred to merely as the "detection pen" hereinafter) 8 due to a stray capacitance between the segment electrode X or the common electrode Y and a tip electrode of the detection pen 8. The voltage induced at the detection pen 8 is amplified in an amplifier 9 and then input to an x-coordinate detection circuit 10 and a y-coordinate detection circuit 11.
The x-coordinate detection circuit 10 and the y-coordinate detection circuit 11 respectively detect an x-coordinate value and a y-coordinate value of a position pointed by the detection pen 8 by detecting a period from the time when the pulse voltage signal is applied to the time when an induction voltage takes its maximum value based on an output signal from the amplifier 9 and a timing signal from the control circuit 7.
It is required to reduce the duration of the coordinate detection period as short as possible in order to allow the duration of the image display period to be as long as possible. Therefore, a segment electrode scanning speed and a common electrode scanning speed in the coordinate detection period are made higher than the drive speed of each electrode in the image display period.
Although the display-integrated type tablet device have the aforementioned advantages, it still has several problems as follows.
[First problem]
In the aforementioned display-integrated type tablet device, there is provided a coordinate detection period for detecting the coordinates at the tip end of the detection pen 8 on the LCD panel 1 in between the image display period of the Nth frame and the image display period of the (N+1)th frame. In contrast to the above, in a normal LCD panel having no tablet function, there is of course existing no coordinate detection period in each frame to continue the image display periods.
Therefore, in applying the display-integrated type tablet device to a personal computer, a word processor, or the like, it is required to develop a display control circuit which can set up a coordinate detection period in place of a conventional display control circuit which cannot set up any coordinate detection period.
The above fact is not problematic when the apparatus employing the display-integrated type tablet device is a new apparatus and fairly large quantity production is expected. However, in the case where the display-integrated type tablet device is mounted to a conventionally manufactured personal computer or the like having no tablet function in a minor change manner and little demand is expected, it is problematic to newly develop a display control circuit in terms of cost and the performance such as reliability of the display control circuit.
Therefore, in the latter case, it is desirable to make the LCD panel function as a display-integrated type tablet device by utilizing the conventionally manufactured display control circuit for the LCD panel having no tablet function and merely incorporating a detection pen, a detection control circuit, a switching circuit, x- and y-coordinate detection circuits, a control circuit, and the like into the LCD panel.
[Second problem]
In the aforementioned display-integrated type tablet device, there is provided a coordinate detection period for detecting the coordinates at the tip end of the detection pen on the LCD panel in between the image display period of the Nth frame and the image display period of the (N+1)th frame in a manner as described hereinbefore. Therefore, the frequency of detecting coordinates in one second (referred to as the "coordinate detection frequency" hereinafter) in the display-integrated type tablet device is equal to the frequency of frames in image display.
The frequency of frames in image display in a personal computer, word processor, or the like has been normally set in a manner as follows.
In the case of a reflection type LCD panel having no back light source and designed as an energy-saving model, for the purpose to avoid flicker due to a fluorescent lamp for room illumination, the frame frequency is determined to be 50 frames per second in a version for use in a district where a 50 Hz power is used, 60 frames per second in a version for use in a district where a 60 Hz power is used, and 72 frames per second in a version for common use in both the districts.
In the case where a fluorescent lamp lit at a high frequency is used for room illumination or in the case of an LCD panel having a back light source, there is no need to take the flicker into account. Therefore, to reduce the power consumption of the LCD panel as low as possible and in view of a compatibility with the display which has been used (e.g., CRT (Cathode Ray Tube) display), the frame frequency is determined to be 60 frames per second or lower.
As described above, the coordinate detection frequency in the display-integrated type tablet device is equal to the number of frames in image display. Therefore, the coordinate detection frequency in the case where the display-integrated type tablet device is adopted for a personal computer, a word processor, or the like, is 72 cycles per second at maximum or 60 cycles per second in almost all the cases.
Although it is theoretically possible to further increase the frame frequency, the frequency of 72 cycles per second can be considered as the higher limit in practical apparatuses in regard to the compatibility with the existing models and the increase of power consumption.
The coordinate detection frequency value (72 cycles per second at maximum, 60 cycles per second normally) of the detection pen is sufficient in pointing a position on the LCD panel by means of the detection pen in the time of menu selection by an icon or when a handwritten letter is input at normal speed to be recognized. However, when a high-speed handwritten pattern is input to be recognized or when a coordinate value is obtained by averaging values detected in several times, a coordinate detection frequency higher than the above-mentioned frequency is desired.
For instance, when a letter or a symbol written at high speed is desired to be detected at a higher rate of recognition, a coordinate detection frequency of 100 cycles per second is sometimes required.
[Third problem]
In the aforementioned display-integrated type tablet device, it is preferred to provide a longer image display period by reducing the coordinate detection period as far as possible to keep a high display image quality. For instance, the duration of the coordinate detection period in the case where the frequency of frames in image display is 60 frames per second is 0.4 mS to 0.8 mS. However, even when the coordinate detection period is thus short, the segment electrode scanning and the common electrode scanning as shown in FIG. 15 must be securely executed. Therefore, the scanning speed of each electrode is very high, when the frequency of the clock signal cp1o reaches 3 MHz.
The segment electrode X or the common electrode Y of the display-integrated type tablet device as shown in FIG. 14 can be expressed by an equivalent circuit consisting of capacitors C and resistors R as shown in FIG. 17 when viewed from the side of the segment electrode drive circuit 3 or the common electrode drive circuit 2.
The capacitors C are electrostatic capacitances between the segment electrodes X and the common electrodes Y which intersect each other with interposition of liquid crystals, where the capacitance reaches 1 pF per pixel in the case of a pixel of 0.33 mm square. The resistors R are the resistances of the segment electrodes X and the common electrodes Y, where the resistance reaches several tens ohms per pixel in the case of a normal LCD panel.
Applying a scanning pulse having a voltage V.sub.5 to the common electrode Y by the common electrode drive circuit 2 in the coordinate detection period is equivalent to applying a scanning pulse having the voltage V.sub.5 to the CR circuit as shown in FIG. 17.
The CR circuit shown in FIG. 17 is a sort of a low-pass filter which deforms the waveform of a scanning pulse applied to the leftmost end of the CR circuit according as the pulse is transmitted rightward and delays the propagation time of the pulse. In the above case, when the scanning speed of the common electrode drive circuit 2 is slow, the propagation delay causes no substantial problem since the delay time is shorter than the coordinate detection time. However, the coordinate detection time has a short duration of 0.4 mS to 0.8 mS in the case of the display-integrated type tablet device to keep high the image display quality, and therefore the propagation delay of the scanning pulse due to the CR circuit cannot be ignored.
Assuming that the frequency of the clock signal cp1o input to the common electrode drive circuit 2 is 3 MHz and the period of from the time when a voltage is induced at the tip electrode of the detection pen 8 to the time when the induction voltage is detected by the y-coordinate detection circuit 11 can be ignored. When the delay of the scanning pulse due to the CR circuit is "0", assuming that the period of from the time when the scanning pulse starts to be applied to a common electrode Y1 to the time when a voltage is induced at the detection pen 8 is T, the y-coordinate at the tip end of the detection pen is expressed by Formula: EQU T.times.(3.times.10.sup.6).times.L (1)
where L is the distance between common electrodes.
The time in which the scanning pulse applied to the leftmost end of the common electrodes Y reaches the rightmost end depends on electrode material, liquid crystal material, the distance between the common electrodes and the segment electrodes which intersect each other, and the like, and takes several microseconds in the case of a normal duty type LCD panel. In the above case, the y-coordinate at the time when the detection pen 8 is located at the leftmost end of the common electrode Y can be expressed by Formula (1) without any problem. However, when the detection pen 8 is located at the rightmost end of the same common electrode Y, the time T in Formula (1) increases by several microseconds from the value when the detection pen is located at the leftmost end. As a result, the y-coordinate value increases despite that the detection pen 8 is located on the same common electrode Y.
What is further disadvantageous is the fact that the dielectric constant of the liquid crystals in the display section differs by about three times from that of the liquid crystals in the non-display section. Therefore, the electrostatic capacitances C of the pixels differ depending on whether they are in the display section or in the non-display section, which results in the variance of the delay time of the CR circuit depending on the content of image displayed even when the detection pen 8 is placed in the same position. In other words, even when the detection pen 8 is held in the same position, the y-coordinate detection value at the tip end of the detection pen varies when the content of image displayed changes.
When, for example, a straight line is drawn in parallel with the common electrode Y by means of the detection pen 8, the resulting straight line is distorted in the presence of such a propagation delay of the scanning pulse in the segment electrode X and the common electrode Y.
Although the displacement of the coordinate detection position due to the propagation delay of the CR circuit can be generally corrected by software means, it is difficult to correct the above-mentioned type of distortion of the coordinate detection position due to the change of the content of image displayed.
Meanwhile, the pulse position of the shift data (so) input to the segment electrode drive circuit 3 which has selected the coordinate detection mode is shifted in the shift register of the segment electrode drive circuit 3 in synchronization with the clock signal cp1o, and the scanning pulses of segment electrode scanning signals x.sub.1 through x.sub.n (an arbitrary segment electrode scanning signal represented by x hereinafter) are successively applied from the output terminals O1 through Om corresponding to the shift position to the segment electrodes X.sub.1 through X.sub.m.
Although the above description is provided for the case where the segment electrodes X are scanned based on the shift data (so) and the clock signal cp1o, the segment electrodes X may be scanned in a manner as follows. In more detail, by using any bit of drive data D.sub.0 d through D.sub.3 d output from the detection control circuit 6 as the shift data sd and using the clock signal cp2d as a sync signal, the segment electrode scanning signal x is output sequentially from the output terminals O1 through Om of the segment electrode drive circuit 3 to the segment electrodes X.sub.1 through X.sub.m.
In the above case, the clock signal cp2d is a clock signal having a cycle in which the segment electrode X is scanned, and is input as the clock signal cp2o to the clock input terminal XCK of the segment electrode drive circuit 3 via the output terminal CP20 of the switching circuit 4.
It is noted that an output terminal EIO2 of the segment electrode drive circuit 3 is the output terminal in the final stage of the shift register, and a pulse signal sio2 having the same pulse width as that of the shift data sd is output from the output terminal EIO2 in a manner as shown in FIG. 13.
The segment electrode scanning signal x is also generated based on the bias power sources V.sub.0 through V.sub.5 supplied from the DC power supply circuit 12.
The coordinate detection period is separated into an x-coordinate detection period and a subsequent y-coordinate detection period. In the x-coordinate detection period, scanning pulses of the segment electrode scanning signal x are successively applied to the segment electrodes X, while in the y-coordinate detection period, scanning pulses of the common electrode scanning signal y are successively applied to the common electrodes Y.
In the above case, the voltage of the scanning pulses of the segment electrode scanning signal x or the common electrode scanning signal y for the electrodes to be scanned among the segment electrodes X and the common electrodes Y (the above-mentioned voltage referred to as the "scanning voltage" hereinafter) is set at the bias power voltage "V.sub.5 " supplied from the DC power supply circuit 12. On the other hand, the voltage of the segment electrode scanning signal x or the common electrode scanning signal y for the electrodes to be not scanned among the segment electrodes X and the common electrodes Y (the above-mentioned voltage referred to as the "non-scanning voltage" hereinafter) is set at the bias power voltage "V.sub.1 " supplied from the DC power supply circuit 12.
A voltage is induced at the detection pen 8 as shown in FIG. 18B due to stray capacitance between the segment electrode X or the common electrode Y and the detection pen 8 as shown in FIG. 18A attributed to the application of the scanning voltage V.sub.5. The voltage induced at the detection pen 8 is amplified in the amplifier 9 and then converted into binary data in a manner as shown in FIG. 18C to be input to the x-coordinate detection circuit 10 and the y-coordinate detection circuit 11.
The x-coordinate detection circuit 10 and the y-coordinate detection circuit 11 detects the x-coordinate value or the y-coordinate value of the position pointed by the detection pen 8 by detecting a time "T" of from the time when the scanning voltage V.sub.5 is applied to the time when the induction voltage takes its maximum value based on an output signal from the amplifier 9 and a timing signal from the control circuit 7.
FIG. 19 shows a timing chart of the segment electrode scanning signal x applied to the segment electrode X of the LCD panel 1 in the x-coordinate detection period.
FIG. 20 is a block diagram of an LCD panel 120, a common electrode drive circuit 121, and a segment electrode drive circuit 122 of the display-integrated type tablet device having the above-mentioned construction when the device is applied to a color display.
The above-mentioned display-integrated type tablet device basically has the same construction as that of the display-integrated type tablet device shown in FIG. 14 except that a filter having a specified light transmission characteristic is provided for each pixel of the LCD panel 120.
When the common electrode Y is placed in a position higher than that of the segment electrode X as viewed from the operator as shown in FIG. 20, an R-color filter is formed in an intersection region of the common electrode Y opposite to the segment electrode (e.g., X.sub.1, X.sub.4, X.sub.7, . . . ) to which the drive pulses of the segment electrode drive signal is applied based on display data of R (Red) color (e.g., display data D.sub.0 o). A G-color filter is formed in the intersection region of the common electrode Y opposite to the segment electrode (e.g., X.sub.2, X.sub.5, X.sub.8, . . . ) to which the drive pulses of the segment electrode drive signal is applied based on display data of G (Green) color (e.g., display data D.sub.1 o). In the same manner, a B-color filter is formed in the intersection region of the common electrode Y opposite to the segment electrode (e.g., X.sub.3, X.sub.6, X.sub.9, . . . ) to which the drive pulses of the segment electrode drive signal is applied based on display data of B (Blue) color (e.g., display data D.sub.2 o).
The surface on which the color filters are formed is not limited to the above-mentioned surface, and when the segment electrode X is placed in an upper position, the color filters are formed on the segment electrode X. Otherwise, the color filters may be formed on a glass substrate on which the electrode placed in the lower position is formed.
When an image on the LCD panel 120 is viewed by reflection light of light entering from outside, there are formed color filters of C (Cyan), M (Magenta), and Y (Yellow) colors being the additive complementary colors respectively of the R, B, and G colors.
In the case of a simple type LCD panel which is not required to present full colors, two types of color filters are formed.
In the LCD panel 120 for color image display, the segment electrode X has a smaller electrode width and a smaller pitch as compared with that of the LCD panel 1 for monochrome image display shown in FIG. 8. In accordance with the above arrangement, there are greater number of segment electrodes X and greater number of output terminals O1 through Om of the segment electrode drive circuit 122.
For instance, in the display-integrated type tablet device for monochrome image display as shown in FIG. 14, the number m of the segment electrodes X is m=640, and the number n of the common electrodes Y is n=480. The pitch of the segment electrodes X is approximately equal to the pitch of the common electrodes Y. However, in the LCD panel 120 for color image display as shown in FIG. 20, the minimum unit of an image to be displayed (referred to as the "picture element" hereinafter) is composed of a combination of three pixels of R, G, and B arranged along the common electrode Y. Therefore, in order to display the same number of 480.times.640 picture elements as in the display-integrated type tablet device shown in FIG. 8, there are required 640.times.3=1920 segment electrodes X.
Furthermore, in the display-integrated type tablet device for color image display, the LCD panel 120 is designed to have approximately the same dimensional aspect ratio as that of the LCD panel 1 for monochrome image display so as to assure a compatibility with the display-integrated type tablet device for monochrome image display.
As a result, the width of the segment electrode X is smaller than that of the common electrode Y in the LCD panel 120 for color image display as shown in FIG. 20.
In the LCD panel 120 for color image display as shown in FIG. 20, the electrode density of the segment electrodes X is approximately three times as high as the electrode density of the common electrodes Y. Therefore, in constructing a display-integrated type tablet device using the LCD panel 120, the following problems exist.
[Fourth problem]
In a display-integrated type tablet device for monochrome image display as shown in FIG. 14, it is required to scan 640 electrodes in the x-coordinate detection period. Therefore, assuming that the frequency of the clock signal cp1d is 3 MHz, the scanning period of one segment electrode X is 0.333 .mu.sec, and therefore the x-coordinate detection period is 0.333.times.640=213.12 .mu.sec.
In contrast to the above, the x-coordinate detection period in the display-integrated type tablet device for color image display having the LCD panel 120 shown in FIG. 20 is 0.333.times.1920=639.36 .mu.sec which is longer by approximately 430 .mu.sec than that of the display-integrated type tablet device for monochrome image display.
As a result, when the number of frames per second of the display-integrated type tablet device for color image display is made equal to the number of frames per second of the display-integrated type tablet device for monochrome image display, the x-coordinate detection period is increased from the period shown in the chart of FIG. 15. The longer the coordinate detection period is, the shorter the image display period is in consequence. The above results in the problem that the duty ratio of image display is reduced to degrade the display image quality.
Furthermore, since the pitch of the segment electrodes X is smaller than the pitch of the common electrodes Y, the scanning speed in the x-direction (shift speed of the segment electrode scanning signal x) is slower than the scanning speed in the y-direction (shift speed of the common electrode scanning signal y) when the segment electrodes X and the common electrodes Y are scanned in synchronization with the clock signal cp1d. Consequently, the time in which the pulses applied to the segment electrode X for inducing a voltage at the detection electrode of the detection pen 8 is approaching the detection electrode is increased, and the time in which the pulse passes below the detection electrode is increased.
FIG. 21 shows waveforms (waveforms relative to a peak value of "1") of voltages induced at the detection electrode of the detection pen 8 in scanning the segment electrodes X and the common electrodes Y having different pitches. As is evident from the chart, the waveform of the voltage induced in the time of scanning the segment electrodes X having the smaller pitch exhibits a wholly gentle curve with a wide foot portion in contrast to the waveform of the voltage induced in scanning the common electrodes Y having the greater pitch.
The fact that the waveform of the voltage induced at the detection pen 8 is gentle means that the rise time "T.sub.1 " of the pulse tends to deviate while being influenced by noise contained in the foot portion of the voltage waveform in the time of converting the induction voltage into binary data by means of a comparator or the like in a manner as shown in FIG. 18C. Therefore, the time "T" to the peak of the voltage waveform calculated based on the rise time "T.sub.1 " tends to deviate, which results in lowering the x-coordinate detection accuracy at the tip end of the detection pen 8.
[Fifth problem]
In the display-integrated type tablet device for monochrome image display and the display-integrated type tablet device for color image display described as above, the coordinates at the tip end of the detection pen 8 are detected by utilizing the electrostatic capacitance between the segment electrode X or the common electrode Y and the detection electrode of the detection pen 8. In the above case, the voltage induced at the detection electrode is proportional to the electrostatic capacitance.
Therefore, when the electrode width is narrow as in the case of the segment electrode X of the display-integrated type tablet device for color image display, the voltage induced at the detection electrode of the detection pen 8 is low. As a result, the accuracy in detecting the x-coordinate at the tip end of the detection pen 8 is lowered.